Optical filters, such as fiber Bragg gratings (FBGs) and the like, are common components in optical fiber-based systems for use in various application fields such as information and telecommunication technologies. FBGs generally consist of patterns of refractive-index variations provided along a section of an optical fiber, which give rise to a high reflectivity in a specific narrowband wavelength region. The strongest reflection of light generally occurs at the Bragg wavelength, whose value depends on the period of the refractive-index variations (i.e., the grating period) and the average effective refractive index of light in the grating region.
Both the grating period and the average effective refractive index vary nearly linearly in response to a tensile load applied to the fiber such that a relative shift in Bragg wavelength also exhibits a nearly linear dependence on the applied tensile load. The Bragg wavelength is generally adjusted by mounting the section of optical fiber along which the optical filter is provided under tension in a packaging assembly, and by controlling the applied tensile load to tune the Bragg wavelength.
The Bragg wavelength of FBG-based optical filters also varies nearly linearly with temperature. To ensure that the spectral response of the FBG remains unaffected when the optical fiber undergoes temperature variations, it is known to package the optical filter in a passive temperature-compensation assembly that acts to control the temperature-induced elongation of the optical fiber containing the FBG. This is usually achieved by fixing the optical fiber to a mechanical structure that imposes a negative elongation (i.e., a contraction) to the fiber when the temperature rises. This contraction of the fiber compensates for the increases of its refractive index with temperature and, thus, allows the Bragg wavelength to be stabilized against temperature fluctuations.
Passive temperature compensation can also be achieved through the principle of differential thermal expansion, which usually involves clamping the fiber containing the FBG to a structure made of materials having different and usually positive coefficients of thermal expansion (CTEs). This structure is arranged such that the CTEs of the different structural elements supporting the fiber cause an overall negative elongation of the fiber when the temperature rises. Typically, the fiber is stretched out at low temperatures and allowed to relax as the temperature increases.
One drawback of optical-filter packaging techniques is that their manufacturing accuracy does not typically lead to a very precise adjustment of the Bragg wavelength. In fact, the assembly process, along with environmental stress screening testing, may induce errors that can cause the Bragg wavelength to deviate from its target value by several picometers (pm) to up to a few hundred pm. While some optical applications can tolerate such deviations, there are several applications where they are unacceptable and other applications that would benefit from increased wavelength accuracy. Currently available packaging techniques either suffer from a lack of precision or rely on active tunable systems to achieve an adequate precision. Although some of these active systems may provide satisfactorily optical performance in some implementations, they tend to be bulky, costly to operate and to require constant power input.
Accordingly, many challenges remain in the development of methods and packaging techniques for fiber-based optical filters that can alleviate wavelength inaccuracies originating from their manufacturing process while providing a simple, compact and passive implementation.